1. Field of the Invention
The present invention relates to a steam turbine for a thermal power plant and, more particularly to moving blades for such a steam turbine.
2. Description of the Related Art
As is generally known, increase in the annulus area of the final-stage moving blades of a steam turbine is essential to increase the capacity of the steam turbine. FIGS. 1 and 2 show the relation between the annulus area for one flow of the final-stage moving blades of a 50/60 Hz tandem compound steam turbine and the range of possible output. FIG. 3 shows the relation between the annulus area for one flow of the final-stage moving blades of a 60 Hz combined-cycle single-casing steam turbine and the range of possible output. In FIGS. 1, 2 and 3, straight lines A indicate possible maximum outputs, and straight lines B indicate possible minimum outputs. As obvious from FIGS. 1 to 3, the necessary annulus area of the final-stage moving blades increases with the increase of output. The maximum annulus area of a well-known final-stage moving blades of a 60 Hz steam turbine is on the order of 8.7 m2 as shown in FIG. 1, and that of a 50 Hz steam turbine is on the order of 14.6 m2 as shown in FIG. 2. Therefore, the possible maximum output of a 60 Hz tandem compound steam turbine is about 1000 MW and that of a 50 Hz tandem compound steam turbine is about 900 MW. The possible maximum output of a steam turbine for a 60 Hz combined-cycle plant is 90 MW. If the output exceeds the possible maximum output, the velocity of the exhaust of the turbine increases, increasing exhaust loss accordingly.
The final-stage moving blades of a 1200 MW class 60 Hz tandem compound steam turbine or a steam turbine for a 400 MW class 60 Hz single-flow combined-cycle plant (with 130 MW class steam turbine) need an annulus area of 9.6 m2 or above as shown in FIGS. 1 and 3. The final-stage moving blades of a 1000 MW class 50 Hz tandem compound steam turbine needs an annulus area of 15.4 m2 as shown in FIG. 2. However, no conventional final-stage moving blades meet such a condition and hence a steam turbine having such a large capacity has not been achieved.
Recently, titanium alloys have widely been used as materials for aircraft structural members, building structural members, sports gear, frames of spectacles and such, and the demand/supply ratio of titanium alloys has increased. However, steels, such as a 12-Cr chromium steel, is superior to titanium alloys in respect of stability of supply and cost. Steels are superior to titanium alloys also in reliability based on the previous results of practical use. Although the final-stage moving blades need an annulus area of 11.5 m2 at the least to reconstruct a 600 MW low-pressure single-casing steam turbine, such a final-stage moving blade cannot be realized by conventional techniques, and hence a high-performance 600 MW class tandem compound low-pressure single-casing steam turbine cannot be realized.
Stresses induced in the moving blade due to the centrifugal force and vibrations acting on the moving blade are difficulties that make the realization of such a high-performance steam turbine impossible. The final-stage moving blades must have a long length and/or must be arranged in a large blade root circle diameter to increase the annulus area. However, such a moving blade increases centrifugal forces that act on an effective blade part, a dovetail and a rotor bore, and stresses induced in various parts of the moving blade increase accordingly beyond the material strength of the moving blade.
Problems that arise when the length of the moving blade is increased to increase the annulus area include the vibration of the moving blade. The rotation of a rotor wheel and the flow of steam passing between the moving blades generate vibrations in the moving blade of a steam turbine. The amplitude of vibrations corresponding to a stress induced in the moving blade increases with the increase of the length of the moving blade or with increase of the flow of steam. Thus, vibration-reducing techniques are very important to develop a moving blade for the final-stage of a large-capacity steam turbine.
Final-stage moving blades having an annulus area greater than that of conventional final-stage moving blades are needed to cope with the recent need for a large-capacity, compact steam turbine. However, restrictions set by centrifugal-force related stress or vibration related stress hinder the current techniques from providing a final-stage moving blade suitable for such a large-capacity, compact steam turbine.
Accordingly, it is an object of the present invention to provide moving blades, specifically final-stage moving blades, having a large annulus area for a low-pressure turbine stage.
Specifically, the present invention is directed to: final-stage moving blades having an annulus area exceeding 9.6 m2 for one flow for a 60 HZ steam turbine; moving blades having an annulus area exceeding 15.4 m7 for one flow for a 50 Hz steam turbine; or moving blades of a steel having an annulus area exceeding 11.5 m2 for one flow for a 50 Hz steam turbine.
The second object of the present invention is to provide a moving blade having a shape capable of preventing the induction of the centrifugal-forth related and vibration related stresses exceeding the allowable strength of the material of the moving blade in the moving blade even if the moving blade has an increased length and arranged in an increased blade root circle diameter.
The third object of the present invention is to provide moving blades for a turbine, having an excellent vibration-damping characteristic capable of damping vibrations generated therein while the turbine is in operation.
To achieve the above objectives, the present invention provides a moving blade assembly for a stage of a low-pressure steam turbine having a raced operating speed of 50 Hz or 60 Hz. The assembly includes a rotor wheel having dovetail slots; and a plurality of moving blades secured to the rotor wheel and arranged as an annular array circumferentially around the rotor wheel, each of the moving blades having dovetails inserted into each of the dovetail slots of the rotor wheel.
The features of the arrangement of the moving blades are as follows: the moving blades are configured so as to meet a condition: (L+3D)xc3x97fxe2x89xa714,500 or Lxc3x97(L+D)xc3x97f2xc3x971.55xc3x97107, where L is an effective length of the moving blade measured in inches, D is a blade root circle diameter of the moving blades measured in inches, and f is the rated operating speed of the turbine measured in Hz; the moving blades are formed of a titanium alloy; each of the dovetails has an Christmas-tree shape and is a curved-axial-entry; the moving blades are configured and arranged so as to meet a condition: 3.5xe2x89xa6P/Wxe2x89xa67.0 or 0.8xe2x89xa6P/Cxe2x89xa61.01, where W is a width of a tip of the moving blade measured along an axial direction of the rotor wheel, P is a pitch of the moving blades measured at the tips of the moving blades and C is a chord length measured at the tip of the moving blade; and all adjacent moving blades of said plurality of moving blades are connected to each other in such a manner that the adjacent moving blades being capable of relative movement with the adjacent moving blades being connected to each other.
If the moving blades meet a condition; Lxc3x97fxe2x89xa72400, the moving blades are preferably formed of an xcex1+xcex2-type titanium alloy.
If the moving blades meet a condition: Lxc3x97fxe2x89xa72700, the moving blades are preferably formed of a near-xcex2-type titanium alloy.
In the event that the moving blades are configured so as to meet conditions: 1.45xc3x97104xe2x89xa7(L+3D)xc3x97fxe2x89xa71.3xc3x97104 and Lxc3x97(L+D)xc3x97f2 less than 1.55xc3x97107, the moving blades may be formed of a martensitic stainless steel having a Cr content in a range of 10 to 18 wt % instead of said titanium alloy.
In a specific embodiment, each of the moving blades has a snubber cover formed integrally with the moving blade; and the adjacent moving blades are connected to each other by contacting the snubber covers of the adjacent moving blades to each other in such a manner that the adjacent snubber covers are capable of relative movement while the adjacent snubber covers contacting to each other, and that the moving blades form a continuous ring around the rotor wheel.
In a specific embodiment, each of the moving blades has a lug formed integrally with the moving blade at a middle portion of an effective part of the moving blade; the adjacent moving blades are connected to each other by a lug-and-sleeve connection including a sleeve and the lug engaged with the sleeve, the connection allowing relative movement between the lug and the sleeve; and the lugs and the sleeves form a continuous ring around the rotor wheel.
Preferably, the moving blade has no through holes formed therein.
It is preferable that the dovetail of each of the moving blades is configured so as to meet a condition: 0.8 less than R/Wr less than 1.2, where R is a radius of a center line of the dovetail as viewed from a radial direction of the rotor wheel, and Wr is a width of the dovetail measured along the axial direction of the rotor wheel.